Panaetius (/pəˈniːʃiəs/; Greek: Παναίτιος; c. 185 - c. 110/109 BC[1]) of Rhodes was a Stoic philosopher. He was a pupil of Diogenes of Babylon and Antipater of Tarsus in Athens, before moving to Rome where he did much to introduce Stoic doctrines to the city. After the death of Scipio in 129 BC, he returned to the Stoic school in Athens, and was its last undisputed scholarch. With Panaetius, Stoicism became much more eclectic. His most famous work was his On Duties, the principal source used by Cicero in his own work of the same name.

Probably through Gaius Laelius, who had attended the lectures of Diogenes and then of Panaetius,[6] he was introduced to the Aemilian Scipio Africanus and, like Polybius before him,[7] gained his friendship.[8] Both Panaetius and Polybius accompanied him on the Roman embassy that Scipio headed to the principal monarchs and polities of the Hellenistic east in 139-138 BC.[9] Along with Polybius, he became a member of the Scipionic Circle.

He returned with Scipio to Rome, where he did much to introduce Stoic doctrines and Greek philosophy. He had a number of distinguished Romans as pupils, amongst them Q. Scaevola the augur and Q. Aelius Tubero the Stoic. After the death of Scipio in spring 129 BC, he resided by turns in Athens and Rome, but chiefly in Athens, where he succeeded Antipater of Tarsus as head of the Stoic school.[10] The right of citizenship was offered him by the Athenians, but he refused it. His chief pupil in philosophy was Posidonius. He died in Athens[11] sometime in 110/09 BC,[1] the approximate year in which L. Crassus the orator found there no longer Panaetius himself, but his disciple Mnesarchus.[12]

With Panaetius began the new eclectic shaping of Stoic theory; so that even among the Neoplatonists he passed for a Platonist.[13] For this reason also he assigned the first place in philosophy to Physics, not to Logic,[14] and appears not to have undertaken any original treatment of the latter. In Physics he gave up the Stoic doctrine of the conflagration of the universe;[15] endeavoured to simplify the division of the faculties of the soul;[16] and doubted the reality of divination.[17] In Ethics he recognised only a two-fold division of virtue, the theoretical and the practical, answering to the dianoietic and the ethical of Aristotle;[14] endeavoured to bring the ultimate object of life into nearer relation to natural impulses,[18] and to show by similes the inseparability of the virtues;[19] pointed out that the recognition of the moral, as something to be striven after for its own sake, was a leading fundamental idea in the speeches of Demosthenes;[20] would not admit the harsh doctrine of apatheia,[21] and, on the contrary, vindicated the claim of certain pleasurable sensations to be regarded as in accordance with nature,[22] while he also insisted that moral definitions should be laid down in such a way that they might be applied by the man who had not yet attained to wisdom.[23]

The principal work of Panaetius was, without doubt, his treatise On Duties (Greek: περὶ τοῦ καθήκοντος) composed in three books. In this he proposed to investigate, first, what was moral or immoral; then, what was useful or not useful; and lastly, how the apparent conflict between the moral and the useful was to be decided; for, as a Stoic, he could only regard this conflict as apparent not real. The third investigation he had expressly promised at the end of the third book, but had not carried out;[24] and his disciple Posidonius seems to have only timidly[25] and imperfectly supplied what was wanting; at least Cicero, who in his books On Duties intended, not indeed to translate, but to imitate Panaetius in his own manner,[26] in the third section of the subject, did not follow Posidonius, but declares that he had completed independently and without assistance what Panaetius had left untouched.[27] To judge from the insignificant character of the deviations, to which Cicero himself calls attention, as for example, the endeavour to define moral obligation,[28] the completion of the imperfect division into three parts,[29] the rejection of unnecessary discussions,[30] small supplementary additions,[31] in the first two books Cicero has borrowed the scientific contents of his work from Panaetius, without any essential alterations. Cicero seems to have been induced to follow Panaetius, passing by earlier attempts of the Stoics to investigate the philosophy of morals, not merely by the superiority of his work in other respects, but especially by the endeavour that prevailed throughout it, laying aside abstract investigations and paradoxical definitions, to exhibit in an impressive manner the philosophy of morals in its application to life.[32] Generally speaking, Panaetius, following Aristotle, Xenocrates, Theophrastus, Dicaearchus, and especially Plato, had softened down the harsh severity of the older Stoics, and, without giving up their fundamental definitions, had modified them so as to be capable of being applied to the conduct of life, and clothed them in the garb of eloquence.[33]

That Cicero has not reproduced the entire contents of the three books of Panaetius, we see from a fragment, which is not found in Cicero, preserved by Aulus Gellius,[34] and which at the same time makes us acquainted with Panaetius's treatment of his subject in its rhetorical aspects.

Panaetius also wrote treatises concerning On Cheerfulness;[35] on the Magistrates;[36]On Providence;[37]On Divination;[17] a political treatise used by Cicero in his De Republica; and a letter to Quintus Aelius Tubero.[38] His work On Philosophical Schools[39] appears to have been rich in facts and critical remarks, and the notices which we have about Socrates, and on the books of Plato and others of the Socratic school, given on the authority of Panaetius, were probably taken from that work.